Ok I know that given how often it's alluded to, this is probably a pretty silly question, but I have to ask it nonetheless because I simply havn't figured it out on my own and I'm currently trying to decide between 2 different burners one which is supposedly very good at scanning but not so good at burning (Lite-On LH-20A1P) and the other which is supposedly quite good at burning but can't scan at all (LG GSA-H42N).

So what exactly is scanning, how does it work, and why is it a valuable thing to be able to do with your burner?

Basically, scans are a way to measure how many errors there are on a disc, so an user can have a reasonable idea about how well (or badly) was burned a disc.

This method it is not an absolute way to determine disc quality, but it is a reasonably reliable method that any user can use at home (i.e. without using really expensive industrial equipments) to check the reliability of a burning.